1. Field of the Invention
This invention relates to ceramic and ceramic-metal articles having a contacting surface exhibiting low coefficient of friction under high vacuum.
2. Description of the Prior Art
Since World War II, lubrication has emerged as a science. Fundamental contributions had been previously made by DaVinci, Coulomb, Reynolds, Amontons, Hardy, Bowden, and Tabor. A number of efforts aimed at trying to understand the fundamental nature of surfaces and their interactions in rubbing, rolling and sliding contact resulted in the development, after World War II, of liquid lubricants, as substitutes for the conventionally used mineral oils, in the aircraft industry. Study of the fundamental and basic structure of organic lubricant compounds became important as a means of understanding the basic mechanism involved in the lubrication of solid surfaces. In addition, the design of synthetic lubricants required understanding the fundamental interactions of solid surfaces with the synthetic lubricants.
Up until the late 1950s, it was extremely difficult to understand the nature of solid surfaces since vacuum technology was not sufficiently advanced to allow evaluations under ultrahigh vacuum conditions. High vacuum technology prior to the late 1950s permitted low pressures of no less than about 10.sup.-6 torr. The solid surface studied must be carefully controlled to avoid surface contamination and results related to the oxide film instead of the ceramic. After the development of ultrahigh vacuum technology it was possible, in the early 1960s, to obtain clean, solid surfaces for study since low pressures of less than about 10.sup.-8 torr (sometimes referred to as ultrahigh vacuum) became possible. It therefore became feasible to study the frictional properties of atomically clean surfaces.
Concommitant with or shortly thereafter the development of this increased capability with respect to the creation and maintaining of high vacuum environments, sophisticated analytical tools became available which permitted analysis of the surfaces under study. Some of the most important tools include low energy electron defraction, Auger electron spectroscopy, and x-ray photo electron spectroscopy.
A considerable amount of research has gone into studying the topography of solid surfaces. The surface profilometer is a useful tool for comparing and identifying solid surfaces. When used to compare one surface with another and show the differences that have taken place in the surface as the result of wear processes, it has been found that the nature of a solid surface often changes with abrasion very markedly from the surface just prior to the initiation of relative motion between two solid surfaces. Metals, for instance steel, have been more commonly studied with respect to surface changes as a result of sliding or rolling contact. Studies have shown that the wear on such surfaces can be characterized as either "adhesive" wear or "abrasive" wear. Abrasive wear is frequently observed with a steel surface in oxygen rich environments in which iron oxide is present on the surface of the steel. Abrasive wear often produces a very smooth surface topography. On the other hand, adhesive wear can produce a very rough surface topography. In contrast to the study of metal surfaces, ceramic and metal-ceramic (cermet) surfaces have been less frequently studied with respect to adhesive or abrasive wear effects. Generally the term cermet is used to describe a ceramic matrix having metallic particles physically dispersed therein. The term is also used to describe the reverse, that is, a metal matrix dispersed therein ceramic materials which impart ceramic-like characteristics to the cermet. Useful representative metals for forming cermets are chromium, copper, cobalt, iron, nickel, lead, molybdenum, and tungsten.
Friction has been defined as the resistance to tangential motion of one solid-state contact surface moving over another. The motion can be rolling or rubbing contact motion. Friction is divided into static and dynamic types. Static friction is the force required to move two solid surfaces or alternatively, the force necessary to break adhesive bonds formed at the interface between two solid surfaces. Dynamic friction is associated with rubbing, rolling, or sliding of one surface over a second surface. Dynamic friction is an average force measurement obtained during such a rubbing, sliding, or rolling process. The term coefficient of friction is used to describe the resistance to tangential motion between two solid surfaces. It is the frictional force divided by the load applied to the two surfaces in contact.
It is well known that two atomically clean, smooth metal surfaces coming into solid-state contact stick together. The sticking together of solid surfaces is often observed when such surfaces are placed in a high vacuum system in which surface oxide films or other contaminants are removed. This sticking together is explained by the development of strong adhesive bonding between the two surfaces. It is significant that small concentrations of oxygen admitted into the vacuum environment bring about marked reductions in the coefficient of friction between two solid-state surfaces. This effect is well known between two iron surfaces; iron is thought to react with the oxygen to form an iron oxide film on the solid-state surfaces thus reducing the "cleanliness" and also the coefficient of friction between the surfaces. This film acts as a lubricant because it insulates the surfaces so as to prevent the adhesion that would normally result between atomically clean, smooth surfaces.
In addition to metallurgical and physical characteristics of solid surfaces having an influence on the adhesion, coefficient of friction, wear, and lubrication of solid surfaces, surface chemistry is important in understanding the frictional characteristics of solid-state surfaces. It has been found that mechanical "working" rolling or rubbing the surface creates an enhanced surface reactivity. That is, the very fact of the mechanical action or activity at the surface results in a heightened chemical reactivity, as compared to the chemical reactivity of the surface under static conditions. For instance, two surfaces in solid-state contact may have the corrosivity of the surfaces enhanced substantially over that which is characteristic of the surfaces under static conditions. Similarly, metals which are under strained environmental constraints are susceptible to heightened chemical reactions. When such reactions occur to produce reaction products on the surface, the solid-state adhesion, friction, and wear properties of the solid surfaces are substantially changed.
Specifically, with respect to the frictional properties of ceramics and metal-ceramics under sliding contact conditions in a high vacuum of about 10.sup.-10 torr, the friction and wear behavior of single-crystal silicon carbide has been studied in contact with itself and in contact with various metals by Miyoshi and Buckley, Friction and Wear Behavior of Single-Crystal Silicon Carbide in Sliding Contact with Various Metals in ASLE Transactions, vol. 22, 3, 245-256 (1978). These investigators found that when loads of 5 to 50 grams were applied to a silicon carbide pin-disc contact at 25.degree. C. under a vacuum pressure of 10.sup.-8 newtons per square meter with a sliding velocity of 3 milimeters per minute and a total sliding distance of 2.5 milimeters that the coefficient of friction was generally constant.
Similar results are provided in NASA Technical Memorandum 81547, Anisotropic Tribological Properties of Silicon Carbide by Miyoshi and Buckley (1981). These investigators found that the coefficient of friction for a silicon carbide pin riding over a silicon carbide surface in vacuum was generally constant at 0.6. Fischer and Tomizawa, Interaction of Tribochemistry and Microfracture in the Friction and Wear of Silicon Nitride, Wear, 105, 29-45 (1985) more recently have found that Si.sub.3 N.sub.4 pins sliding in air on Si.sub.3 N.sub.4 plates provide a coefficient of friction which initially starts at a relatively low value (0.4-0.6) and thereafter reaches a stable value of 0.7-0.8. The higher the load, the faster the stable value is reached.
These results and the generally understood knowledge in the prior art, especially with respect to the sticking together of two atomically clean, smooth surfaces, would not suggest that the friction under vacuum of an atomically clean ceramic body riding on a ceramic flat surface could be subject to a reduction in friction. Thus, after wear track conditioning, the coefficient of friction of the ceramic surface wear track remains low thereafter under high vacuum conditions.